1. Field of Invention
This invention provides a photochromic glass highlight mask and carrier especially useful in photo-enlarging positive color transparencies, using photochromic glass which darkens only under blue and shorter wavelengths of radiation.
2. Description of the Related Art
Photographic transparencies, including positive color transparencies, may incorporate a far broader range of contrast than is feasible for photoprinting. Highlight areas (those of greatest transparency) are particularly difficult; and control or "holding back" of highlights by highlight masks is a useful technique. A related but more comprehensive technique is contrast reduction--masking to lessen contrast not merely in the highlight or thin areas of the transparency, but instead to lessen contrast over the print area. For this related purpose, black-and-white film masks are customarily used; however attempts have been made to use photochromic glass, as hereinafter referred to.
Masks made from black-and-white film effect general contrast reduction over the entire area of the transparency. In making these, a photo transparency to be printed (whether positive or negative, color or black and white) is first placed above and in contact with a photosensitive black-and-white film; and an exposure is made.
Disadvantages of this practice include: Making such a negative film mask requires removal to a dark room, developing, and drying for roughly two hours at room temperature, because drying under heat would shrink the film. Should the mask density be too little or too great, this procedure must be repeated. After drying, the mask must be assembled between the enlarger carrier glasses in precise registration with the transparency to be printed; even using pins for alignment, perfect registration is hardly possible.
Efforts heretofore made, to make such general contrast-reduction masks using photochromic glass, are believed to have been unsuccessful. Since earlier photochromic glass responded substantially only to ultraviolet radiation, one effort, shown in U.S. Pat. No. 3,680,956 to Custer, requires that (a) only ultraviolet light be used both for making such a mask, (b) in using the mask in photoprinting, only ultraviolet light be projected; and (c) since ordinary glass will filter out most ultraviolet radiation, only quartz glass be used for all glass elements in the enlarger.
Another effort to make general contrast-reduction masks of photochromic glass, is set forth in the recently issued patent to Banks, U.S. Pat. No. 4,758,502 dated July 19, 1988. Banks may have intended such contrast reduction only for black-and-white photographs; he states incorrectly (Col. 7, starting with line 10): "It is in the nature of the brightest parts of a photograph to be fairly white and the darkest parts to be fairly black, so the contrast mask will form properly for these areas with little regard for the colour sensitivity of photochromic body, The color range in its sensitivity is important only in the areas of medium optical density . . . " Whatever this may mean, it makes clear that his purpose was not to mask only the highlights.
Banks specifies, for making his mask, a "multiple wavelength white light" which is to have "a colour temperature of at least 4000 K. more preferably at least 5000 K." He requires that the photochromic glass must darken in response to "the majority of wavelengths in the visible spectrum" and not fade in response to any visible radiation. To the best of the present inventor's knowledge, no photochromic glass known in the United States has such response and and non-fading characteristics. Banks suggests that certain experimental glass has been made in the United Kingdom which will meet his criteria, provided that the mask be made in a cold room at a different location than the enlarger; and that even the enlarger must operate at a temperature not to exceed 70 degrees F. Air-cooled enlargers now in professional use operate at temperatures in the range of 80-90 degrees F. For all these reasons the Banks' teachings do not appear to be practicable.